In the manufacture of semiconductor devices, it is common to place small solder balls upon the substrate of the semiconductor device in a ball grid array and then reflow the solder balls in an oven to provide a series of electrical connections on the substrate. Currently, there are several different approaches for placing solder balls to form ball grid arrays onto semiconductors and other electrical devices.
In one approach, an array of solder balls is picked up with a vacuum head. Each solder ball is held by a separate vacuum nozzle in the vacuum head. The vacuum head then places the solder balls on the semiconductor substrate and releases the solder balls thereon.
In another approach, a mask is placed over the semiconductor substrate. The mask has an array of openings formed therethrough corresponding to the desired pattern of electrical connections on the substrate. A quantity of solder balls is then spread across the mask with an air knife or a squeegee. Some of the solder balls fall into and are captured by the openings in the mask thereby positioning the solder balls in the desired pattern on the substrate.
In still another approach, a transfer substrate is formed with an array of indentations corresponding to the desired pattern of electrical connections on the substrate. The array of indentations is then filled with solder balls. The semiconductor substrate is brought face down into contact with the array of solder balls lying on the transfer substrate. The solder balls are then reflowed and metallurgically bonded to the semiconductor substrate.
In some applications, as many as 1000 solder balls about 0.020 inches to 0.030 inches in diameter are placed on a semiconductor substrate in an area of about four square inches. As a result, the large number of small sized solder balls makes it sometimes difficult to consistently place a full array of solder balls on a semiconductor substrate with current machinery.
The present invention provides an apparatus for placing an array of solder balls on a semiconductor substrate which is more reliable in the placement of the solder balls than previous approaches. The present invention apparatus includes a carrier plate having a series of holes therethrough. Each hole is capable of holding a solder ball. At least a portion of a first pattern of protrusions on a ball placement head is aligned with a first pattern of solder balls held by the carrier plate. The protrusions push the first pattern of solder balls through the holes in the carrier plate onto the substrate.
In preferred embodiments, a solder ball feed fills the carrier plate with solder balls. The feed includes a compliant wiping element for wiping excess solder balls from the carrier plate. A second pattern of protrusions on a pattern head is aligned with some of the solder balls held by the carrier plate for pushing a second pattern of solder balls from the carrier plate so that only the first pattern of solder balls remains held by the carrier plate. Each protrusion on the ball placement and pattern heads is a pin which is self-aligning with a corresponding hole in the carrier plate.
A first sensing system senses whether all the required holes in the carrier plate contain a solder ball after being filled by the solder ball feed. The first sensing system includes a vision device and a light positioned behind the carrier plate for back lighting the carrier plate. A second sensing system senses whether the carrier plate contains solder balls only in the first pattern after the pattern head pushes the second pattern of solder balls from the carrier plate. The second sensing system includes a vision device, a light positioned behind the carrier plate for back lighting the carrier plate for detecting whether the carrier plate holds solder balls only in the first pattern, and a light positioned in front of the carrier plate for front lighting the carrier plate for detecting any stray solder balls on top of the carrier plate.
The carrier plate in one preferred embodiment includes a film sandwiched between a first plate portion and a second plate portion. The series of holes in the carrier plate pass through the first plate portion, the film and the second plate portion. The holes in the first and second plate portions are dimensioned to allow passage of the solder balls therethrough while the holes through the film are dimensioned to prevent passage of the solder balls through the film by gravity while allowing passage through the film when pushed by the ball placement head.
The first sensing system, pattern head, second sensing system and ball placement head are arranged in a circular path at a first sensing station, a pattern head station, a second sensing station and a ball placement station, respectively. A rotatable carousel having a support finger for supporting the carrier plate transfers the carrier plate to each station. The carousel provides for simultaneous operation at each station resulting in higher through put.